An ion is formed when an atom undergoes a loss or gain of electrons.
A cation is formed when an atom loses the most loosely bound electron from its outermost shell. The atom acquires a positive charge and becomes an ion (a cation).
M (g) → M+ (g) + e–
A cation is smaller than its atom. On the removal of an electron, the positive charge of the nucleus acts on lesser number of electrons than in the neutral atom and thus greater pull is exerted by the nucleus, resulting in a smaller size of the cation.
An anion is bigger than its atom because on receipt of an electron in the outermost orbit the number of negative charges increase and it outweighs the positive charges, Thus the hold of the nucleus on the shells decrease resulting in an increase in the size of the anion.
M (g) + e– → M– (g)
Trends in Group
In the main groups, the ionic radii increase on descending the group. It is due to the addition of extra shell at each step.
Trends in Period
There is a decrease in the ionic radii of the positive ions on moving from left to right across a period in the periodic table. It is due to the increase in the number of charges on the nucleus and also due to the increase in the charge on the ion.
The ionic radii of the negative ions, also decrease on moving from left to right across a period. This is partly due to increase in the number of charges on the nucleus and also due to the decreasing change on the ion.